1. Field of the Invention
The present invention relates to a voltage regulator that produces from an input voltage a predetermined output voltage to be supplied to a load, and relates particularly to the optimization of an overcurrent protect circuit incorporated in such a voltage regulator.
2. Description of the Prior Art
FIG. 3 is a circuit diagram showing an example of the configuration of a conventional voltage regulator. The conventional regulator IC 1′ shown in this figure is built by sealing into a single package an output circuit (a P-channel MOS transistor P1, an operational amplifier OP1, a direct-current voltage source E1, and resistors R1 and R2) for producing from an input voltage Vi a predetermined output voltage Vo to be supplied to a load Z1 and an overcurrent protection circuit (a P-channel MOS transistor P2, a resistor R3, and an N-channel MOS transistor N1) for preventing overcurrent in the output circuit.
It is true that, in the regulator IC 1′ configured as described above, the predetermined output voltage Vo produced from the input voltage Vi is supplied to the load Z1 connected externally to the output terminal of the regulator IC 1′. Moreover, the overcurrent protection circuit prevents the output current Io from reaching the capacity limit of the transistor P1 even when, as immediately after electric power starts being supplied to the regulator IC 1′, the output current Io flows at a dash into a bypass capacitor C1.
However, in the conventional regulator IC 1′, the level at which the overcurrent protection circuit detects overcurrent is fixed at the time of the fabrication of the circuit. This makes it impossible to cancel factors such as individual variations in the characteristics of the circuit components, the influence of stress occurring when the regulator IC 1′ is packaged or mounted on a circuit board, and fabrication-associated variations in the load Z1 connected to the regulator IC 1′.
Thus, in the conventional regulator IC 1′, the level at which to detect overcurrent is set rather high (generally 1.5 times or more as high as the upper limit of the actually used range of the output current Io, see FIG. 4) with a margin secured so that the level at which to detect overcurrent does not happen to be lower than the upper limit of the actually used range of the output current Io.
As a result, with the overcurrent protection circuit mentioned above, it is possible to prevent the output current lo from reaching the capacity limit of the transistor P1 when, as immediately after electric power starts being-supplied to the regulator IC 1′, the output current Io flows at a dash into the bypass capacitor C1, but this is achieved at the cost of unnecessarily great power loss as indicated by hatching in FIG. 4.
Moreover, in anticipation of such power loss, the regulator IC 1′ needs to be supplied with electric power from a power supply device with a rather high power supply capacity. This increases the size and cost of the power supply device.
Incidentally, there is available a regulator IC (not shown) having a sense resistor, for detecting the output current, connected outside the IC so that overcurrent is prevented on the basis of the voltage across the sense resistor. With this type of regulator IC, by setting the resistance of the sense resistor appropriately, it is possible to vary the level at which to detect overcurrent. This apparently helps solve the problem described above. In reality, however, the sense resistor has a very low resistance, and thus tends to be influenced by variations in connection resistances and other factors. Thus, the sense resistor is difficult to match with the circuit components within the IC. This makes it extremely difficult to satisfactorily correct for a variation in the level at which to detect overcurrent.
As a result, even in the regulator IC configured as described above, the level at which to detect overcurrent needs to be set rather high with a margin secured so that the level at which to detect overcurrent does not happen to be lower than the upper limit of the actually used range of the output current. Thus, it is not possible to satisfactorily solve the problem described above. In addition, the sense resistor, connected externally, hampers the scaling-down and cost reduction of the appliance incorporating the regulator IC.